Wind-Powered Car Travels Downwind Faster Than the Wind

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Wind-Powered Car Travels Downwind Faster Than the Wind

The basic concept.

Rick Cavallaro and his friends have built a wind-powered vehicle that travels downwind faster than the wind, solving a riddle that can start fights.

The unusual wind-powered car hit a top speed 2.86 times faster than the wind during one recent run, a feat that – depending upon your perspective — is either the result of hard work or the same voodoo responsible for Ryan Seacrest's hair.

"People's intuition is extremely strong on the topic," Cavallaro, an aerodynamicist and avid kitesurfer and paraglider, said. "There are literally thousands of pages of debate on internet forums about the topic."

His explanation, then, sounds deceptively simple.

"If you're on a bike and you're going downwind, you don't feel any wind anymore at all," he said. "You lose the power of the wind when you reach the wind speed, because there is no relative wind at that point."

When he isn't obsessing over aerodynamics, Cavallaro works for Sportvision, the company that created the FoxTrax hockey puck – aka the "glowing puck" – and the yellow first-down line shown on telecasts of football games. Cavallaro's boss at Sportvision, world-class sailing navigator Stan Honey, turned him on to the DWFTTW question. Working with a hang-gliding buddy, Cavallaro did the math and built a model to prove DWFTTW is possible. Skeptics weren't convinced.

"I thought people would say, 'That's cool,' but they didn't. They said, 'Wow, you're an idiot.' So we decided to build a full-size one. That's when we approached a couple of sponsors."

Cavallaro lined up help from Google and Joby Energy and set to work with the San Jose State University aero department on an ultralight, four-wheeled vehicle with a 17-foot-tall propeller. The vehicle is made mostly of foam and mimics the aerodynamics of a Formula 1 race car. But it's the propeller that is key to how it is possible to travel downwind faster than the wind. It's also the source of the biggest misunderstandings about how the vehicle works.

"Skeptics think that the wind is turning the prop, and the car is turning the wheels, and that's what makes the car go," Cavallaro said. "That's not the case. The wheels are turning the prop. What happens is the prop thrust pushes the vehicle."

The wheels turn the prop, which turns the vehicle's wheels, which turn the prop, which turns the vehicle's wheels. Cavallaro knows what you're thinking.

"It sounds like a perpetual motion machine – but you've got the wind as an external power source," he said.

Cavallaro said building a transmission capable of transferring power from the wheels to the prop was almost as hard as convincing skeptics that the vehicle would work. It took longer than a year and a lot of trial and error to make it work.

"You've got to come up with a transmission that can handle those loads, even though it's not at a high horsepower," he said. "You break some things, and then you build bigger."